In the quest for improved performance, electronic circuits are becoming denser and devices smaller. For example, the most common gate dielectric in metal oxide semiconductor field effect transistors (MOSFET) has been SiO2. However, as the thickness of SiO2 approaches 20 Å, substantial problems appear, including large leakage currents through the gate dielectric, long term dielectric reliability, and difficulty of manufacture and thickness control. One solution to the above problems is to use thick films of materials such as aluminum oxide which have a dielectric constant larger than SiO2. Thus, the physical thickness of the gate dielectric can be large while the electrical equivalent thickness relative to SiO2 films can be scaled. The electrical equivalent thickness, teq, of a high dielectric constant material, relative to SiO2, for example, may be calculated using the formula:teq=tphy(∈SiO2/∈high K)where tphy is the actual thickness of the substitute metal oxide gate dielectric, such as aluminum oxide and ∈SiO2 and ∈high K are the dielectric constants of SiO2 and the metal oxide gate dielectric film, respectively.
Similar problems are encountered in scaling capacitors in memory devices. As the circuits become denser and the devices smaller, a material with a higher capacitance such as aluminum oxide is necessary to store adequate charge in the capacitor. Aluminum oxide has a dielectric constant of 10, which is more than double the dielectric constant of SiO2 (∈=4) and is thus an attractive material for replacement of SiO2 in transistors and capacitors.
However, CVD of aluminum oxide from alkyl aluminum CVD precursors, such as trimethylaluminum seems to be inherently contaminated with carbon; see R. S. Ehle, et al. J. Electron. Mater. Vol. 12, 1983, p. 587. Similar carbon contamination is observed in aluminum oxide deposited from alkylaluminum alkoxides. XPS survey spectrum and Auger depth profile of aluminum oxide deposited at 400° C. on Si with an alkylaluminum alkoxide (triethyldialuminum tri-sec-butoxide) show carbon contamination throughout the film. See, for example, T. M. Klein, et al., Appl. Phys. Lett., Vol. 75 1999, p. 4001. Similar carbon contamination would be expected in films grown with similar alkylaluminum alkoxides as described, for example, in U.S. Pat. No. 6,037,033. Aluminum oxide has been deposited using aluminum β-diketonates, such as aluminum tris(2,4-pentanedionato) and aluminum tris(tetramethylheptanedionato). However β-diketonates are known to undergo complex decomposition pathways which may lead to carbon incorporation in the film. Low deposition temperatures and addition of H2O as an oxidant are recommended to obtain aluminum oxide films; See, J. S. Kim, et al., Appl. Phys Lett., 62(7) 1993 P. 681. However, low deposition temperatures and water results in a porous film with excess OH. Deposition of aluminum oxide from AlCl3 and H2 and CO2 has been described in U.S. Pat. No. 4,097,314. An incubation period of about 30 seconds is necessary before film growth occurs which results in an uncertainty in controlling growth rates of thin films (less than 1000 Å). Residual Cl and H contaminates are present in the film. Additionally AlCl3 is highly corrosive generating HCl as a byproduct.
Deposition of aluminum oxide from aluminum alkoxides is known and deposition of aluminum oxide at temperatures less than 500° C. has been described; See, for example, J. A. Aboaf, J. Electrochem. Soc. 1967, Vol. 114(9), p. 948; J. Fournier, Mat. Res. Bull., 23 31 (1988); and H. Mutoh, J. Electrochem. Soc. 122, 987 (1975). Deposition of aluminum oxide with aluminum isopropoxide deposited on Si at 740° C. has been reported by S. S. Yom et al., Thin Solid Films, 213, 72, 1992. However, as described in U.S. Pat. Nos. 5,431,734, 5,540,777, 5,648,113, and 5,728,222, aluminum alkoxides are known to isomerize during heating and delivery resulting in unreproducible precursor delivery and film growth. Moreover the aforementioned U.S. patents describe methods and apparatuses for depositing aluminum oxide from aluminum isopropoxide utilizing FTIR monitoring of the aluminum isopropoxide vapor from a bubbler to improve reproducibility. Despite the improved reproducibility, the '734, '777, '113 and '222 patents suffer from the disadvantage of using conventional bubbler technology.
The other previous work with aluminum alkoxides patents also have utilized conventional bubbler technology which involves a carrier gas bubbled through a neat (i.e., without solvent) precursor at an elevated temperature. The conventional bubbler technology relies on a consistent vapor pressure of the precursor to deliver a uniform precursor flux to the film. In addition, because vapor pressure is directly related to temperature, conventional bubbler technology suffers from the disadvantages of needing to maintain a bubbler temperature with minimal variation during a run and from run to run. Fluctuations in precursor flux are known to result in variable film growth rates. Solid compounds are known to sinter and change surface area over time, resulting in nonuniformity in film growth rates from run to run. Sintering is not a problem for liquid precursors, but over time the liquid precursors may degrade from the thermal cycling and thermal load placed on the precursor. Additionally, at elevated temperatures, decomposition processes are accelerated. As described in U.S. Pat. Nos. 5,431,734, 5,540,777, 5,648,113, and 5,728,222, elevated temperatures and thermal cycling of aluminum alkoxide during vaporization in a conventional bubbler contributes to premature degradation of the aluminum alkoxide over time. Aluminum alkoxides change their chemical state by ligand rearrangement, cluster formation, or oxidation. Aluminum alkoxides are known to react with water or oxygen inadvertently introduced into the bubbler through inadequately purified carrier gases bubbled through the precursor, air leaks, or water and oxygen adsorbed on the bubbler walls. Furthermore, aluminum alkoxides are known to isomerize during heating resulting in many different species with varying vapor pressures, with the result that consistent vapor pressure is difficult to achieve with conventional bubbler technology.
An additional difficulty of depositing aluminum oxide by CVD is that aluminum oxide exists in a number of morphological forms. The aluminum oxide polymorph obtained is critically dependent on deposition conditions, such as the CVD precursor, oxidant, growth temperature and pressure, and substrate. Additionally, the only thermally stable modification of aluminum oxide is α-alumina (α-Al2O3, corundum or sapphire). All other polymorphs are metastable and irreversibly converted to α-Al2O3 at sufficiently high temperatures.
An additional difficulty of fabricating devices on silicon with aluminum oxide deposited on Si by CVD is the potential growth of an interfacial oxide layer during deposition or during post deposition processing. See for example, T. M. Klein, et al., Appl Phys. Lett., Vol 75 1999, p. 4001.
In view of the drawbacks with prior art processes of forming aluminum oxide films, there is a continued need for developing a new and improved method for depositing aluminum oxides which avoids each of the above mentioned prior art problems.